(a) Field of the Invention
The present invention relates to a liquid crystal display and a device of driving a light source therefor.
(b) Description of Related Art
Display devices used for monitors of computers and television sets generally include self-emitting display devices such as organic light emitting displays (OLEDs), vacuum fluorescent displays (VFDs), field emission displays (FEDs), and plasma panel displays (PDPs), and non-emitting display devices such as liquid crystal displays (LCDs) requiring external light source.
An LCD includes two panels provided with field-generating electrodes and a liquid crystal (LC) layer having dielectric anisotropy and interposed therebetween. The field-generating electrodes that are supplied with electric voltages generate electric field across the LC layer, and the light transmittance of the liquid crystal layer varies depending on the strength of the applied field, which can be controlled by the applied voltages. Accordingly, desired images are displayed by adjusting the applied voltages.
The light for an LCD is provided by lamps equipped at the LCD or may be a natural light. When employing the lamps, the brightness on a screen of the LCD is usually adjusted by regulating the ratio of on and off durations of the lamps or regulating the current flowing in the lamps.
The lamps for the LCDs usually include fluorescent lamps driven by an inverter. The inverter converts DC voltage into AC voltage and applies the AC voltage to the lamps to be turned on. The inverter adjusts luminance of the lamps according to a luminance control signal to control the luminance of the LCD. In addition, the inverter feedback controls the voltages applied to the lamps based on the currents of the lamps.
Recently, an external electrode fluorescent lamp (EEFL) receives attention as a substitute of a cold cathode fluorescent lamp (CCFL) since EEFL is relatively cheap and facilitates the parallel driving. In detail, EEFL includes external electrodes attached at both ends of a discharge tube and thus the external electrodes and the discharge tube play a role of ballast capacitors, thereby requiring no separate ballast capacitor, which is necessary for CCFL having inner electrodes. Accordingly, the parallel driving of EEFL is easy.
Since EEFL has a symmetrical structure that the ballast capacitors are disposed at both ends of the tubes, the electrodes at the both ends of the tube are required to be supplied with equal voltages, while the voltage difference between the electrodes are required for generating a current in the tube. Accordingly, so called floating type driving that applies two voltages having equal magnitude and opposite polarities to the electrodes is usually employed. In other words, two voltages having a phase difference of 180 degrees are applied to the opposite electrodes.
In order to generate such voltages, a secondary coil of a transformer for generating voltages for driving the lamps is usually divided into two equivalent sub-coils and the node between the sub-coils is grounded via resistors. The inverter performs the above-described feedback control based on the current flowing in one the two sub-coils.
However, since adjacent lamps are supplied with the same periodical voltages that may cause constructive electromagnetic interference therebetween and may interfere the electric field applied in the LC layer.